Semiconductor structures, such as integrated circuits, become more complicated in the dimensions and shapes of pattern features. Accordingly, there exists an increasing need in providing accurate measurements of full 3-dimensional structures, and in enabling these measurements to be applied to structures progressing on a production line, i.e. automatic inspection (metrology, defect detection, process control, etc.) of patterned structures.
Current metrology techniques heavily rely on test structures (which are typically produced in the scribe lines of a wafer) and on attempt to represent the process behavior inside the structure (which is not always successful). However, measuring directly the features inside the structure has a significant benefit as it allows both the relevance that test structures sometimes lack and the ability to map the changes across the structure.
Currently several metrology methods exist for the measurement of 2-dimensional (lines) and 3-dimensional structures. These methods can be roughly divided into four main groups including optical imaging techniques, beam scanning techniques, probe-based microscopy, and “non-imaging” optical techniques, usually termed scatterometry or optical critical dimension (OCD) measurement.
Optical imaging techniques are based on creation of a direct image of the region (area) of a sample. These techniques are in most cases no longer relevant for accurate geometrical measurements of such patterned structures as semiconductor wafers, because the features size of a pattern is much smaller than the wavelength used for imaging. This limitation may sometimes be overcome by utilizing aerial image of a mask taken prior to magnification down to the wafer, as done in some inspection tools (steppers).
Beam scanning techniques are based on scanning a given area of a sample with a focused beam of particles, collecting any kind of radiation produced by interaction between the beam and the sample (usually secondary particle emission), and using intensity (or other parameter) of the collected radiation to create a 2-dimensional image of the sample. Such techniques include, for example, SEM (Scanning Electron Microscopy) and Hellium Ion Beam Microscopy.
Probe-based microscopy utilizes a probe (tip) which is brought in close vicinity with the sample (such as for example in AFM—Atomic Force Microscopy) and scans a line or an area of the sample. Signals from the probe or, more often, feedback control signals (which are used to keep the probe at a constant distance from the sample) are used for creation of a 2-dimensional image of the sample.
Scatterometry or OCD techniques are based on measurement of diffraction from a repetitive structure on a sample (grating), having periodicity in either one or two directions, and reconstruction of the geometrical parameters of a unit cell of a pattern through solving the inverse problem, and fitting a model to the measurement results. Here, a measurement spot contains many periods of the repetitive structure, hence a measurement represents the average parameters across the measurement spot.
It should be noted that optical imaging techniques, beam scanning techniques and probe-based microscopy can all be implemented as scanning techniques and can create an image by scanning probe with high resolution (i.e. sensitive to a small part of the sample at a time) over the sample. As for the scatterometry or OCD techniques, they have several advantages, such as high speed and repeatability, but they usually suffer from a significant handicap which is the long setup time required before a measurement can be performed. This issue is more severe in case of 3-dimensional structures as they become more complex, because the number of parameters becomes larger and the diffraction calculation becomes longer.
One of the known approaches to circumvent the above issues is by combining information from additional sources, such as CD-SEM or AFM, for example as described in U.S. Pat. No. 6,650,424 assigned to the assignee of the present application. According to this technique scatterometry and SEM measurements are applied to a structure, and measured data indicative of, respectively, the structure parameters and lateral pattern dimensions of the structure are generated. The entire measured data are analyzed to enable using measurement results of the scatterometry for optimizing the measurement results of SEM and vice versa.